Pneumatic rubber tires are conventionally prepared with a rubber tread which can be a blend of various rubbers which is typically reinforced with carbon black.
In one aspect, rubbers are evaluated, selected and blended for a purpose of achieving desired tire tread properties and particularly a balance of tire tread characteristic properties, mainly, rolling resistance, traction and wear.
For various applications utilizing rubber including applications such as tires and particularly tire treads, sulfur cured rubber is utilized which contains substantial amounts of reinforcing filler(s). Carbon black is commonly used for such purpose and normally provides or enhances good physical properties for the sulfur cured rubber. Particulate, precipitated silica is also sometimes used for such purpose, particularly when the silica is used in conjunction with a coupling agent. In some cases, a combination of silica and carbon black is utilized as reinforcing fillers for various rubber products, including treads for tires.
It is important to appreciate that, conventionally, carbon black is considered to be a more effective reinforcing filler for rubber tire treads than silica, if the silica is used without a coupling agent.
Indeed, at least as compared to carbon black, there tends to be a lack of, or at least an insufficient degree of, physical and/or chemical bonding between the silica particles and the rubber elastomers to enable the silica to become a reinforcing filler for the rubber for most purposes, including tire treads, if the silica is used without a coupler. While various treatments and procedures have been devised to overcome such deficiencies, compounds capable of reacting with both the silica surface and the rubber elastomer molecule, generally known to those skilled in such art as coupling agents, or couplers, are often used. Such coupling agents, for example, may be premixed, or pre-reacted, with the silica particles or added to the rubber mix during the rubber/silica processing, or mixing, stage. If the coupling agent and silica are added separately to the rubber mix during the rubber/silica mixing, or processing stage, it is considered that the coupling agent then combines in situ with the silica.
In particular, such coupling agents may generally be composed of a silane which has a constituent component, or moiety, (the silane portion) capable of reacting with the silica surface and, also, a constituent component, or moiety, capable of interacting with the rubber, particularly a sulfur vulcanizable rubber which contains carbon-to-carbon double bonds, or unsaturation. In this manner, then the coupler may act as a connecting bridge between the silica and the rubber and thereby enhances the rubber reinforcement aspect of the silica.
In one aspect, the silane of the coupling agent apparently forms a bond to the silica surface, possibly through interaction with silanol groups on the silica surface, and the rubber interactive component of the coupling agent apparently interacts with the rubber. Usually the rubber interactive component of the coupler is temperature sensitive and may tend to combine with the rubber during the higher temperature sulfur vulcanization of the rubber composition and after the silane group of the coupler has reacted with the silica. However, partly because of typical temperature sensitivity of the coupler, some degree of interaction may occur between the rubber-interaction component of the coupler and the rubber during initial rubber/silica/coupler mixing stage(s) and, thus, prior to the subsequent vulcanization of the rubber composition.
The rubber-interactive reactive group component of the coupler may be, for example, one or more of groups such as mercapto, amino, vinyl, epoxy, and sulfur groups, preferably a sulfur or mercapto moiety and more preferably sulfur.
Numerous coupling agents are taught for use in combining silica and rubber, such as, for example, silane coupling agents containing a polysulfide component, or structure such as, for example, bis-(trialkoxysilyalkyl) organosilane polysulfides containing from 2 to about 8 sulfur atoms in a polysulfide bridge such as, for example, bis-(3-triethoxysilylpropyl)tetrasulfide, trisulfide or disulfide. In the case of the disulfide, if in a high purity form, some free sulfur, or sulfur donor, may desirably be added with the coupler in a rubber mixing step to enhance an overall effect of interaction with the elastomer, or rubber.
For silica reinforced tire treads, U.S. Pat. No. 5,066,721, in its Comparative Test Example 1 in Table 3 (column 15), discloses the use of solution polymerization prepared SBR containing 50 parts silica for a tire tread. Table 4 (column 17) illustrates the tire preparation. U.S. Pat. No. 5,227,425 discloses the use of a solution polymerization prepared SBR which is silica reinforced and in which is preferenced over an emulsion polymerization prepared SBR. U.S. Pat. No. 4,519,430 discloses a silica rich tire tread which contains solution or emulsion SBR, optionally with polybutadiene rubber and/or polyisoprene rubber together with a mixture of silica and carbon black, with silica being required to be a major component of the silica/carbon black reinforcing filler.
Styrene/isoprene/butadiene terpolymer rubbers have been disclosed for use in tire treads. For example, U.S. Pat. No. 5,159,020 discloses such a terpolymer rubber (SIBR) and use thereof in a tire tread composition. It is disclosed therein that such tire tread can be composed of a blend of the SIBR and at least one other rubber such as natural rubber, polybutadiene rubber, styrene/butadiene copolymer rubber, isoprene/butadiene copolymer, styrene/isoprene copolymer, and 3,4-polyisoprene rubber. It is disclosed therein that the blend can contain silicas.
It is intended that all of such aforesaid patents are incorporated herein by reference.
The term "phr" where used herein, and according to conventional practice, refers to parts of a respective material per 100 parts by weight of rubber, or elastomer.
In the description of this invention, the terms "rubber" and "elastomer" where used herein, unless otherwise prescribed, are used interchangeably. The terms "rubber composition", "compounded rubber", and "rubber compound" where used herein, unless otherwise prescribed, are used interchangeably to refer to rubber which has been blended or mixed with various ingredients or materials and such terms are well known to those having skill in the rubber mixing, or rubber compounding, art.
The Tg of a polymer, particularly an elastomer, as used herein unless otherwise prescribed, refers to its glass transition temperature which can conventionally be determined, for example, by a differential scanning calorimeter at a heating rate of 15.degree. C. per minute to an observed transition of the temperature versus time curve. It is understood that such Tg determination is well known to those having skill in such art.